1. Field of the Invention
The invention relates to a backlight module, and more particularly, to an edge type backlight module.
2. Description of the Related Art
Backlight modules of conventional liquid crystal displays use mostly cold cathode fluorescent lamps (CCFLs) as light sources. However, cold cathode fluorescent lamps contain mercury, consume too much power and have short lifetime. Hence, the current trend is toward replacing cold cathode fluorescent lamps with light emitting diodes (hereinafter referred to as LEDs) when designing the light sources of backlight modules. LEDs are durable and contamination-free. Red, green, blue monochromatic LEDs have advantages, such as a single wavelength and a wide color gamut, and thus resultant liquid crystal display (LCD) images are brightly colored and delicately outlined. However, an LED is a point light source and thereby is unlikely to provide uniform illumination for a light emission surface of a backlight module, and in consequence granular hot spots are visible in front of the backlight module.
From an optical point of view, uniform illumination emerges as light travels further away from a point light source. Inasmuch as the appeal of LCDs lies in the compactness thereof, an LCD whose uniform illumination-providing surface source is fabricated at the cost of a thickened backlight module is necessarily bulky and thereby is hardly in line with the trend of LCD products. In order to enhance the practicability of LED backlight modules, U.S. Pat. Nos. 6,607,286, 6,679,621 and 7,438,444 disclose a lens structure configured for lateral emission of LED light with a view to providing uniform illumination.
For example, referring to FIG. 1, U.S. Pat. No. 6,607,286 discloses a lens cap 1 mounted on an LED 2. The lens cap 1 includes a concave inner surface 10 and a serrate outer surface 11. The LED 2 is covered by the inner surface 10 of the lens cap 1. The light beams emitted by the LED 2 penetrate the inner surface 10 before being refracted by the outer surface 11 and then exiting laterally. Among these light beams, oblique beams have to undergo total internal reflection once before being refracted and exiting.
However, the above-mentioned lens cap is of particular shape and therefore expensive.